1. Field of the Invention
The present invention is directed to a method of producing a thermoelectric semiconductor which is used, particularly, as a thermoelectric cooling element.
2. Discussion of the Background
As a conventional thermoelectric semiconductor compound, a crystal is well known which is obtained by solidifying a thermoelectric semiconductor material, such as a bismuth-tellurium series, through one of the Bridgman technique and Zone melting crystallization. However, such a one-way crystallized thermoelectric semiconductor crystal has cleavage along planes determined by bonding surfaces of tellurium-tellurium, which results in it being brittle. Thus, employing such a one-way crystallized thermoelectric semiconductor as a thermoelectric cooling element inevitably causes problems in reliability and/or mechanical strength.
In order to overcome such serious problems from the viewpoint of practical use, especially for improving mechanical strength, Japanese Patent Laid-open Print No Sho.62-264682 (published in 1987 without examination) provides a new thermoelectric semiconductor which is produced by the following steps: powdering a crystalline body of a thermoelectric semiconductor, and sintering the resultant powdered material while applying a force thereto along one direction. Due to the fact that the thermoelectric semiconductor is applied with the force along only one direction, the cleavage planes are oriented in a direction perpendicular to the force application direction. Thus, applying en electric current along each of the resultant cleavage planes utilizes or activates the electrical isotropy of the crystalline body of the thermoelectric semiconductor, in addition to achieving an improved mechanical strength which results from the sintering.
However, in this method, before performing the foregoing sintering under pressure, the powdered thermoelectric semiconductor is placed in a mold device and hot pressed, which results in that orientation of each cleavage plane is restricted or limited, thereby inhibiting the electrical isotropy of the bismuth-tellurium series thermoelectric semiconductor as an original property thereof.
By contrast, Japanese Patent Laid-open Print No. Hei. 10-112558 published in 1998 without examination provides a method of producing a thermoelectric semiconductor having the following steps: powdering a thermoelectric semiconductor material, and sintering such a powdered thermoelectric semiconductor during extrusion. In the resultant sintered thermoelectric semiconductor body, cleavage planes are oriented along the extruding direction, thereby improving the electric conductivity of the thermoelectric semiconductor body.
However, even the foregoing newly published sintering methods as described above fail to provide a remarkable improvement in the electric conductivity of the thermoelectric semiconductor, and the performance index is at most less than 3.5, which is not satisfactory from the view point of practical use.
Thus, there is a need for a further developed method of producing a thermoelectric semiconductor which is excellent in electric conductivity, and correspondingly performance index, and which is mechanically strong.